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Chapter 10 Nuclear Changes. What is Radioactivity?. Certain isotopes of many elements undergo a process called radioactive decay During radioactive decay, the unstable nuclei of these isotopes emit particles, or release energy, to become stable isotopes
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What is Radioactivity? • Certain isotopes of many elements undergo a process called radioactive decay • During radioactive decay, the unstable nuclei of these isotopes emit particles, or release energy, to become stable isotopes • The released energy and matter is referred to as nuclear radiation • After radioactive decay, the element changes into a different isotope of the same element or into an entirely different element
What is Radioactivity? • Recall that isotopes of an element are atoms that have the same number of protons but different numbers of neutrons in their nuclei • Different elements are distinguished by having different numbers of protons
What is Radioactivity? • Four types of nuclear radiation • Alpha particles • Beta particles • Gamma rays • Neutrons
What is Radioactivity? • Alpha particles • Made of 2 protons and two neutrons • Same as the helium nuclei • Positively charged • More massive than any other type of nuclear radiation • Do not travel far through materials • Barely pass through a sheet of paper • Can ionize other atoms thus losing energy
What is Radioactivity? • Beta particles • Often fast-moving electrons but may also be positively charged particles called positrons • Positrons have the same mass as electrons • Neutrons, which are not charged, decay to form a proton and an electron • The electron, which has a very small mass, is then ejected at a high speed from the nucleus as a beta particle • Pass through a piece of paper , but most are stopped by 3 mm of aluminum or 10 mm of wood • Not as massive as alpha particles • Can ionize other atoms thus losing energy
What is Radioactivity? • Gamma rays • Not made up of matter and do not have an electric charge • Form of electromagnetic energy • Consist of energy packets called photons • Like light and X-rays • Have more energy than light or X-rays • Can ionize and cause damage in matter • Can penetrate up to 60 cm of aluminum or 7 cm of lead • Greater damage to health
What is Radioactivity? • Neutron radiation • No charge • Therefore, do not ionize matter • Therefore, they are able to travel farther through matter than alpha and beta particles • A block of lead about 15 cm thick is required to stop most fast neutrons
What is Radioactivity? • Anytime that an unstable nucleus emits alpha or beta particles, the number of protons and neutrons change • Example: radium-226 (an isotope of radium with the mass number 226)changess to radon-222 by emitting an alpha particle
What is Radioactivity? • Gamma decay changes the energy of the nucleus • No change in the atomic number of the element
What is Radioactivity? • Beta Decay: • Loses electron • Atomic number changes (increase of one) • Mass number before and after the decay does not change • The atomic number of the product nucleus increases by one (neutron changes into a proton) • Example: carbon-14 changes into nitrogen-14
What is Radioactivity? • Alpha Decay: • Loses 2 protons and 2 neutrons (helium nuclei) • Both atomic mass and number change’
What is Radioactivity? • What do the colors mean? • Zone 1 counties have a predicted average indoor radon screening level greater than 4 pCi/L (picocuries per liter) (red zones)Highest Potential • Zone 2 counties have a predicted average indoor radon screening level between 2 and 4 pCi/L (orange zones)Moderate Potential • Zone 3 counties have a predicted average indoor radon screening level less than 2 pCi/L (yellow zones)Low Potential
What is Radioactivity? • Radioactive Decay Rates • Half-Life: time in which half of a radioactive substance decays • Last from a nanosecond to billions of years • Used to diagnose medical problems • Detector follows the element as it moves through the patient’s body • Predict the age of fossils/rocks
What is Radioactivity? • Radium-226 has a half-life of 1,599 years. How long will 7/8 of a sample of radium-226 take o decay? • Fraction remaining 1/8 • Amount of sample remaining after 3 half-lives • ½ x ½ x ½ = 1/8 • 3 half-lives X 1,599 years/half-life = 4,797 years
What is Radioactivity? • Radioactive decay is exponential decay • Example: Carbon-14
Nuclear Change • The stability of a nucleus depends on the nuclear forces that hold the nucleus together. • These forces act between the protons and the neutrons • Like charges repel, so how can so many positively charged protons fit into an atomic nucleus without flying apart? • Strong nuclear forces cause neutrons and protons to attract one another—this attraction is much stronger than the electric repulsion between protons
Nuclear Change • Nuclei with more than 83 protons are always unstable • These nuclei will always decay and, in the process, release large amounts of energy and nuclear radiation
Nuclear Change • Nuclear fission: process of splitting heavier nuclei into lighter nuclei • Neutrons and energy are released • Some mass will be changed into energy • The equivalence of mass and energy is explained by the Theory of Relativity (Einstein) • Energy= mass X (speed of light)2 = mc2 • Under ordinary conditions this change does not happen spontaneously
Nuclear Change • Neutrons released by fission can start a chain reaction (one reaction triggers another and so on) • Continuous series of nuclear fission reactions
Nuclear Change • Critical mass: the minimum mass of a fissionable isotope that provides the number of neutrons needed to sustain a chain reaction • In nature the concentration is low • Chain reactions can be controlled to be used to generate electricity • Control rods regulate fission by slowing the chain reaction
Nuclear Change • Nuclear Fusion: energy can be obtained when very light nuclei are combined to form heavier nuclei • Example: • Sun: energy is formed primarily when hydrogen nuclei combine releasing energy • Large amount of energy is needed to start a fusion reaction • Reason is that all nuclei are positively charged and repel one another with electric force • Energy needed to overcome this force (Stars: the extreme temperature overcomes this force)
Nuclear Radiation Today • We are continually exposed to radiation from natural sources (sun, soil, rocks, plants)(80%) • Human-made sources (computer monitors, smoke detectors, X-rays, etc.)(20%) • Measured in units called rems (1rem = 1,000 millirems) • Dental X-ray= 1 millirem • Safe limit for workers is about 5,000 rems per year
Nuclear Radiation Today • Exposure varies from location to location, activities (jobs/recreational), habits (smoking), home to home (smoke detectors: emit alpha particles)
